1. Field of the Invention
The present invention relates to a moveable headrest suitable for use in automotive vehicles, and specifically to a moveable headrest incorporating therein a ratchet mechanism through which the headrest is adjustable back and forth.
2. Description of the Prior Art
As is generally known, a headrest has often a manual forward-and-back adjustment, so as to be above to tilt the upper portion of the headrest back and forth. In order to change the angle of tilt of the headrest, the moveable or tiltable headrest traditionally accommodates therein a ratchet mechanism. One such moveable headrest employing a ratchet mechanism has been disclosed in Japanese Utility Model Second Publication (Jikko Heisei) No. 3-39078. FIGS. 6-10 show a structure almost similar to a moveable headrest as disclosed in the Japanese Utility Model Second Publication No. 3-39078. The structure of the conventional headrest is hereinbelow explained briefly.
Referring to FIGS. 6 to 10 and specifically to FIGS. 6 to 8, a substantially doughnut shaped conventional moveable headrest 10 having a central opening 12 includes a pair of headrest stays 13 engageable with the upper portion of the seat back, a lower headrest frame shaft 15 constructing a part of a headrest frame 11 and fixed to the headrest stays 13, and an upper moveable or tiltable headrest frame 16 mechanically linked to the lower frame shaft 15 through a ratchet-and-pawl mechanism accommodated in a base member 22 serving as a ratchet base. The doughnut shaped conventional moveable headrest 10 is often called a see-through type moveable headrest since this type of headrests can ensure a better rear view of the vehicle by virtue of its central opening. The headrest 10 also includes a pad 14 usually made of foamable or expandable synthetic resin, preferably made of foamed urethane. As illustrated by the phantom line of FIG. 6, the pad 14 is generally formed integral with the headrest frame 11 by way of injection of foam liquid into the interior of the outer skin constructing the contour of the headrest. The ratchet base 22 is rotatably supported by the lower frame shaft 15 and fixedly connected to the upper tiltable frame 16. The ratchet-and-pawl mechanism includes a toothed ratchet 24 having teeth 24a and a pawl 25 mounted on a pawl bracket 26. By means of the above-noted ratchet-and-pawl mechanism, the ratchet base 22, which is fixedly connected to the upper frame 16, is engageable with or disengageable from the lower frame shaft 15. As seen in FIG. 6, the headrest frame 11 is comprised of the lower frame shaft 15 fixed to the stays 13, the substantially reversed-U shaped upper moveable frame 16, and left and right ratchet bases 22 each constructing a ratchet-and-pawl casing 21. As appreciated from FIG. 6, each ratchet-and-pawl casing 21 is comprised of the ratchet base 22 and a ratchet cover 23 provided to hermetically cover the opening end of the ratchet base 22. As best seen in FIG. 8, the upper moveable frame 16 is traditionally fixed onto the upper inside wall surface of the respective ratchet base 22. As shown in FIG. 7, the base 22 is formed with a circular through-opening 22a through which the end 15a of the lower frame shaft 15 is rotatably fitted into the ratchet-and-pawl casing 21. As illustrated in FIG. 8, the toothed ratchet 24 is fixedly connected to the base 22, whereas the pawl bracket 26 is fixedly connected to the end 15a of the lower frame shaft 15. The pawl 25 is pivotally mounted on the pawl bracket 26 by way of its pivot shaft 28. The pivot shaft 28 is actually guided by an essentially circular arc-shaped guide groove 27 integrally formed on the inside wall of the ratchet cover 23. As seen in FIG. 7, a cam plate 29 is rotatably provided on the end 15a of the lower frame shaft 15 and disposed between the base 22 and the pawl bracket 26 to be engageable with the pawl 25. In a conventional manner, the pawl 25 is usually biased to engage with the tooth 24a of the ratchet 24 by means of a spring 30 hanged between the pawl 25 and the pawl bracket 26. When the headrest 10 has been rotated in its foremost position, as illustrated by the arrow F of FIG. 8, the lower end of the spring 30 moves across the central axis (serving as a fulcrum or a support shaft of the cam plate 29) of the end 15a of the lower frame shaft 15 and thus the bias of the spring 30 acts to disengage the pawl 25 from the tooth of the ratchet 24. Under these conditions, if the headrest 10 is returned to the backmost position as illustrated by the arrow B of FIG. 8, the pawl 25 is brought into engagement with the cam plate 29 and as a result the pawl 25 is also brought into engagement with the tooth 24a of the ratchet 24. With the forward motion of the headrest 10, intermittent tilt motion of the headrest is produced by way of a to-and-fro motion of the pawl against the toothed ratchet 24. Assuming that the vehicle occupant will try to move the headrest 10 backward again when the headrest 10 reaches a desired tilt position while driving the pawl 25 in a to-and-fro motion, the headrest can be held at the desired tilt position by way of the locking action of the ratchet-and-pawl mechanism. In FIG. 8, reference sign 31 denotes a return spring hanged between the right-hand side of the toothed ratchet 24 fixed onto the base 22 and the tab-like portion of the cam plate 29 for constantly biasing the upper moveable frame 16 in the backward direction. As explained above, the conventional moveable headrest 10 is so designed that its locking state is released by rotating the headrest in the foremost position F, and that a desired tilt position is adjustable by moving the headrest from the backmost position B to the desired position, and that the headrest is held at the desired position irrespective of any backward motion of the headrest by the locking action of the ratchet-and-pawl mechanism.
In the above-noted conventional headrest 10, when the ratchet base 22, the cam plate 29 and the pawl bracket 26 are assembled onto the end 15a of the lower frame shaft 15, there are several problems, as detailed later in accordance with schematic assembly drawings, namely FIGS. 7, 9 and 10.
Referring now to FIGS. 7, 9 and 10, and specifically to FIG. 9, the end 15a of the lower frame shaft 15 is formed into a small-diameter portion 32 so as to define a stepped portion. The stepped portion is formed with a pair of opposing cut-outs 33 having a substantially D-shape in cross-section, as appreciated from a cross-sectional area 36 illustrated by hatching. As seen in FIG. 10, the pawl bracket 26 is fixed onto the top end of the small-diameter portion 32 of the lower frame shaft 15 by welding. As seen in FIG. 9, the cut-out of the stepped portion provides a pair of opposing substantially T-shaped flat surfaces 33a. The ratchet base 22 and the cam plate 29 are rotatably assembled on the end 15a of the lower frame shaft 15 in such a manner as to be rotatably supported on the outer peripheral surface of the small-diameter portion 32 except the T-shaped flat surface 33a. A washer 35 is interleaved between the radially-extending shoulder 33b of the stepped portion and the base 22, so as to prevent the inner axial movement of the base 22 by way of abutment of the side wall surface of the washer 35 and the radially-extending shoulder 33b of the stepped portion. To prevent the rotational movement of the washer 35 relative to the shaft end 15a (the small-diameter portion 32 having a substantially barrel shape in cross-section), the washer 35 is formed with a pair of opposing substantially D-shaped portions 37 each fitted onto the substantially T-shaped flat surface 33a. After the shaft end 15a is inserted into the barrel-shaped central opening of the washer 35, the through-opening 22a of the base 22, the circular opening of the cam plate 29 and the circular bore 26a of the pawl bracket 26 in that order, the small-diameter portion 32 of the shaft end 15a is fixedly connected to the pawl bracket 26 by welding, as shown in FIG. 10. With the shaft end 15a inserted into the circular bore 26a of the pawl bracket 26, a pair of opposing substantially D-shaped apertures 38 are defined between the inner peripheral surface of the circular bore 26a and the above-noted T-shaped flat surface 33b, owing to the presence of the cut-outs 33. Reference sign 39 denotes a welded metal zone. In such a conventional connecting structure of the pawl bracket 26 onto the shaft end 15a of the lower headrest frame shaft 15, weld spatter which will necessarily take place when welding the pawl bracket 26 onto the shaft end 15a, often enters into the interior of the ratchet-and-pawl mechanism. The weld spatter, can prevent smooth operation of the ratchet. In addition, when the pad 14 of the headrest 10 is formed by foaming expandable synthetic resin, preferably foamed urethane, it is possible for the foam liquid enter into the interior of the ratchet-and-pawl mechanism through the aperture defined between the shaft end 15a and the barrel-shaped opening of the washer 35 and through the aperture defined between the shaft end 15a and the through-opening 22a of the base 22. As appreciated, the urethane foam, which is unintendedly formed in the ratchet-and-pawl mechanism, may prevent smooth operation of the ratchet-and-pawl mechanism. Furthermore, since the ratchet base 22 and the cam plate 29 are rotatably supported on a partial outer peripheral surface of the shaft end 15a owing to the cut-outs 33, such a conventional headrest may be insufficient with respect to a reliability, a mechanical strength, and a durability. Also, the conventional moveable headrest requires an additional part such as the washer 35, and thus a number of parts for the headrest tends to be increased. Moreover, machining required for the D-shaped cut-outs 33 is troublesome, thus increasing production costs of the headrest assembly.